Recently, much concern has been concentrated on developing display devices using the electrochromic characteristics and the development of a novel conductive polymeric material having good processability, stability, response time, etc. has been treated as a very important research project. Now, two types of soluble conductive polymer are known, that is poly(3-alkyl thiophene) and polyaniline.
Among the 3-alkyl thiophenes, octyl substituted thiophene has been found to have good solubility. Octyl thiophene is oxidation-polymerized and washed with-methanol, to give a polymer soluble in chloroform, toluene, etc. Poly(octyl thiophene) can be doped by electrochemical or chemical method after solution casting. Recently, Macdiarmid et al. and Li et al. proposed manufacturing method for a solvent-soluble polyaniline. However, this kind of polymer is difficult to polymerize, and oxidation stability thereof is not sufficiently good for being applied to display devices.
Characteristics of electrochromic display using conductive polymers include absorption spectrum, optical density change, color changing speed, lifetime, etc. Oxidation and reduction reaction of conductive polymer are characterized by doping-dedoping reaction. Electrochemical doping reaction can be explained in the following equation. EQU Pol+mX.sup.n- .fwdarw.Pol.sup.m+ .multidot.m.sup.n- +ne.sup.--
In this equation, Pol is a neutral polymer before doping, X.sup.n- is an anion, Pol.sup.m+ is an oxidized polymer, ne.sup.- represents consumed electrons during the reaction and m and n are integers representing valence.
In the above equation, the colors of the polymer before and after oxidation are different. The response time of the display device is determined by the speed of oxidation and reduction of the conductive polymer in the electrochromic display using this property. The stability is determined by the number of reversible repeatitions of the oxidation and reduction. The change of color when electric potential is changed is directly related to the current change, and the response time of the conductive polymer is concerned with the thickness of the polymer, applied electric potential and polymer structure. Generally, devices having a response time shorter than 100 msec are known to be applicable as display devices electrochromic display.
Manufacturing methods of display device using material having electrochromic characteristics include thermal evaporation (WO.sub.3), sputtering (WO.sub.3), anode oxidation (Ir.sub.2 O.sub.3), sublimation (phthalocyanine) and deep coating (soluble polymer). The methods for forming a conductive polymer film on electrodes are anode oxidation and deep coating. In practice, since most conductive polymers are not soluble in solvents, the anode oxidation method is used. In this case, the thickness of the film can be adjusted by using the amount of electricity. However, only films having a thickness of 300 nm or less are used as for display devices.
The lifetime is the most important factor to be considered for the conductive polymers and it is determined by measuring the time until the electric characteristic reaches to about 50% of the original one. Mostly, the lifetime is about 1000 cycles in the absence of oxygen. Polypyrrole and polythiophene are known to be have stable lifetimes and recently, a polyaniline has been found to have a life time of about 10.sup.4 cycles. However, more development is still needed.
The present inventors completed the present invention after performing repeated experiments to solve the abovementioned problems and to synthesize soluble conductive polymer showing even better characteristics than conventional conductive polymers in view of oxidation-reduction and stability to oxygen.